During the colonial and early frontier periods of the United States, the two main external security threats came from which groups?

The correct choice identifies Native Americans and the British as the two main external security threats during the colonial and early frontier periods of the United States. This is accurate because throughout the colonial era, European settlers faced significant challenges from Indigenous peoples who resisted encroachment on their lands, leading to conflicts such as King Philip's War and various skirmishes and wars across the frontier.

Moreover, the British, as the colonial authority, were involved in numerous military actions and diplomatic maneuvers that created tensions. Their policies often brought settlers into direct conflict with Native American tribes, especially as settlers continued to push westward. The British military presence and interest in maintaining control over the colonial territory added another layer of conflict, particularly during events leading up to the American Revolution.

In contrast, while the French and Spanish were also colonial powers in North America, they did not pose the same immediate threat to the settlers in the regions that would later become the United States during the specified period as much as the British did. Hence, the pairing of Native Americans and British captures the most relevant external security threats of that time.